The present invention relates to a method of coating a flexible support with a coating solution, and more particularly to a method of coating a flexible support with a magnetic solution to form a magnetic recording medium or the like.
The term "flexible support" as used herein is intended to mean a flexible sheet or web made of plastic film, paper, polyolefin-coated paper, or an aluminum or copper sheet. The flexible support may be one having a based coat. These flexible supports may be coated with magnetic solutions, photographing photosensitive coating solutions, and other coating solutions to form magnetic recording media, various photographing films, photographic papers, etc.
Heretofore a variety of coating methods, such as a roll coating method, a gravure coating method, a roll-coating-plus-doctoring method, an extrusion coating method, and a sliding bead coating method, have been employed to coat long flexible supports with coating agents.
Of these coating methods, the extrusion coating method has been extensively employed recently (see, Japanese Patent Application Publications Nos. 45528/1982 and 7306/1979, and Japanese Unexamined Published Patent Application (OPI) No. 189059/1983). A specific feature of the extrusion coating method resides in that a quantitative solution supplying pump capable of providing solution continuously at a predetermined flow rate is used to supply a magnetic-material-dispensed solution, namely, a coating solution, to the slit of the coating head, thereby coating the flexible support with the coating solution. However, the method is disadvantageous in that, in the widthwise direction of the support, the flow rate of the coating solution tends to change because of a loss of pressure in the slit, and accordingly the coat formed on the support is not uniform.
In order to solve this problem, a method has been proposed in the art in which, in the case where the coating solution has the properties of a Newtonian fluid, the slit is tapered in the widthwise direction to thus compensate for the loss of pressure in the slit. However, this method is not suitable for a magnetic layer coating solution because the magnetic layer coating solution greatly affects the electromagnetic conversion characteristics. That is, because the solution has a thixotropic nature, the viscosity of the coating solution changes in the pocket (coating solution pool) from which the coating solution is supplied to the slit of the coating head.
With respect to a coating solution such as a magnetic solution having a thixotropic nature, another method has been known in the art in which a rotary cylinder (or a rotor) is disposed in the pocket to shear the coating solution to thereby maintain the viscosity substantially constant in the widthwise coating direction.
In the case where the extrusion-type coating method is employed to coat a flexible support with a magnetic coating solution, in order to make the coat's thickness uniform in the widthwise direction, the above-described two methods may be employed in combination so that the viscosity in the widthwise direction is made substantially constant by the provision of the rotary cylinder in the pocket and the loss of pressure is compensated by tapering the slit in the widthwise direction.
The present inventors have found that the support still cannot be entirely uniformly coated with the coating solution even by employing these methods together, and thus have conducted intensive research on the cause of the nonuniform coating. As a result of this research, it has been found that the cause is that the pocket of the extrusion type coating apparatus is designed so that the part of the coating solution supplied into the pocket located near the solution supplying point is immediately extruded into the slit. That is, in the above-described conventional method, when the coating solution is supplied into the slit through the pocket, the coating solution near the solution supplying point (i.e., the coating solution receiving inlet) is delivered into the slit substantially without shearing by the rotor. Accordingly, in the coating solution flowing in the slit, the viscosity of the part of the coating solution which is near the solution supplying point of the pocket is different from the viscosities of the other parts. Thus, the coat on the support is not uniform in thickness.
The present invention have conducted more intensive research on the coating method and found that the above-described difficulties can be eliminated by the provision of a coating method according to the invention.